This invention relates to the field of atomization of liquid metals, to produce metallic powders. The invention also relates to the field of cryogenic gases, and provides a system and method for producing a stream of cold gas, the temperature and pressure of the stream being very precisely regulated.
Metal powders are useful in various applications. For example, in the manufacture of printed circuit boards, conductive layers are applied to a substrate in the form of metal powder. If the particles of the powder are too coarse, conductors of the circuit pattern may become short-circuited. To maximize the line density, and to increase the efficiency and yield of the manufacturing process, one needs a metal powder having small, fine, spherical particles.
Metal powders are also useful in applying a uniform metallic coating to a surface, such as by flame spraying or welding. As in the case of printed circuit boards, a uniform coating requires small, spherical, and uniform particles.
Still another application of metal powders is in metal injection molding. In this process, metal powder is mixed with a plastic material and is formed into a shaped article, the particles of the powder becoming fused together with the application of heat. Again, the results of this type of process are most favorable when the particles are small, spherical, and uniform.
Metal powders can also be used for other purposes, such as for soldering and sintering.
Methods of making metal powders have been known in the prior art. A metal powder can be made by directing a pressurized gas, at ambient temperature, towards a liquid metal. The liquid metal is atomized by the gas, and cools to form a powder. The gas is preferably inert, or relatively inert, to prevent oxidation of the metal. The preferred gas is nitrogen, which remains substantially inert throughout a wide range of temperatures.
It has also been known to use a cryogenic liquid, instead of a gas, as the agent which atomizes the liquid metal.
The present invention uses a cold gas to atomize the liquid metal, to form a metal powder. A major problem with such use of cold gas is in the need to control accurately the pressure and temperature of the gas. Such control is necessary to allow precise control of the distribution of particle sizes, and to control the configuration of the particles. It has been found necessary that the pressure fluctuations be less than about 1 psi, and the temperature fluctuations should be less than about .+-.2.degree. F.
Although cryogenic fluid delivery systems have been known for a long time, it has proven difficult to provide a cold gas stream having the above degree of consistency. Examples of dispensing systems of the prior art are shown in U.S. Pat. Nos. 4,909,038, 4,715,187, 4,336,689, 4,961,325, and 4,570,578. Other systems of the prior art include heaters which vaporize specific volumes of liquefied gas, and which use additional trim heaters to achieve desired gas temperatures. None of the above-mentioned systems provides the precision of control of temperature and pressure required in the liquid metal atomization process.
Another problem in the production of metal powders is the appearance of multiple "phases". That is, when a two-component alloy is melted and then slowly cooled, one component may solidify first, causing localized regions of increased concentration of that component. The separated components may manifest themselves as streaks, or dendrites, in the particles of the finished powder. This effect makes the particles less spherical and less homogeneous, and should therefore be minimized.
The present invention solves the above-described problems by providing an apparatus and method which produces a consistent cold gas stream, and which can be used to atomize liquid metals. The apparatus is simple, economical, and reliable, and provides a stream of gas which fulfills the temperature and pressure criteria specified above. The invention is not limited to use in liquid metal atomization, but can be used in any system or process which requires a consistent cold gas stream.